The present invention relates to photovoltaic devices, and more particularly to flexible photovoltaic devices composed of non-single-crystal semiconductors.
Photovoltaic devices such as solar cells and photodetectors are capable of converting solar radiation into usable electrical energy. However, the devices of this type have the large problem to be overcome that the generating cost is very large as compared with other means for generating electric energy. The main reasons are that the efficiency of utilizing a semiconductor material constituting the body of the device is low and much energy is required in producing the semiconductor material. In recent years, it has been proposed to use amorphous silicon as a semiconductor material, and this proposal has been considered as a technique having a possibility of overcoming the above problems. The amorphous silicon can be produced inexpensively and in large quantities by glow discharge decomposition of silicon compounds such as silane and fluorosilane. The amorphous silicon so prepared has a low average density of localized states in the band gap, namely not more than 10.sup.17 eV.sup.-1 cm..sup.-3, and impurity doping of N type or P type is possible in the same manner as in the preparation of crystalline silicon.
A representative prior art solar cell using amorphous silicon is one prepared by forming a transparent electrode on a glass substrate capable of transmitting visible rays, forming a P-type amorphous silicon layer, an undoped amorphous silicon layer and a N-type amorphous silicon layer on the transparent electrode in that order by glow discharge, and providing an ohmic contact electrode on the N-type layer. If light enters the P-type layer, undoped layer and N-type layer through the glass substrate and transparent electrode, electron-hole pairs are mainly generated in the undoped layer. They are attracted by the PIN junction internal electric field created by the above-mentioned layers and are collected at the electrodes to generate a voltage between the both electrodes. In solar cells of such a type, the open-circuit voltage is about 0.8 V, and accordingly the cells are not used as they are as a power source for equipment requiring a large power source voltage.
In order to overcome this drawback, it is proposed to raise the voltage by forming separate plural generating zones on a single substrate and contacting the respective generating zones in series, as disclosed in Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 107276/1980. However, the solar cells prepared according to this proposal still have the drawbacks that a large surface area is required due to low conversion efficiency, and that series connection of multiple devices is necessary because the voltage per single device is only about 0.8 V. Also, the solar cell has no flexibility due to the use of a rigid substrate such as glass. On the other hand, flexible solar cells are required in the field of electronic machines because of the development of flexible printed boards and film type liquid crystal displays.
A solar cell using a polymer thin film rich in flexibility and heat resistance, such as polyimide, as a substrate is disclosed in Japanese Unexamined Patent Publication No. 149489/1979. However, the use of the polymer film as a substrate has the disadvantages that the substrate is curled during the deposition of the amorphous silicon and flat solar cells are not obtained, and because the substrate is not uniformly heated due to the deformation during the deposition non-uniform amorphous silicon layers are formed.
It is an object of the present invention to provide a flexible photovoltaic device.
A further object of the invention is to provide a flexible and heat resistant photovoltaic device.
Another object of the invention is to provide a flexible, small-sized photovoltaic device having a plurality of generating zones on a single substrate.
Still another object of the invention is to provide a flexible integrated solar cell capable of generating an increased open-circuit voltage.
These and other objects of the present invention will become apparent from the description hereinafter.